When ventilatory support or protection of the airway is indicated, tracheal tubes have been the device of choice. The traditional tracheal tube ("breathing tube", "endotracheal tube" or "ETT") is placed into the patient's trachea through the mouth or nose to assist with breathing. It consists of a length of extruded, flexible PVC (or silicon rubber, or other rubber) tubing that has a connector on its proximal end to hook up to a ventilating means. Near the distal end is an inflatable, circumferential cuff or balloon that seals against the walls of the trachea when it is inflated. This seal between the cuff and tracheal wall allows positive pressure ventilation of the lungs and prevents vomited stomach contents from leaking down and soiling the lungs from above.
The alternative method of tracheal tube placement is through a tracheostomy (a surgical hole through the skin of the neck and directly into the trachea). This type of tracheal tube is called a "tracheostomy tube". The distal end and the inflatable cuff sealing means of both the tracheal and tracheostomy tubes are identical.
Tracheal tubes are the essential means of controlling the exchange of gases in the critically ill or surgical patient. The tracheal tube establishes a closed system whereby the gas pressure in the airway distal to the inflated cuff can be controlled, allowing external positive pressure mechanical ventilation of the patient's lungs.
The standard, currently used tracheal tubes have cuffs that resemble inflatable doughnuts near the distal end. The cuffs are usually made of a thin film of PVC or whatever material the tube is made of. The cuffs are manually inflated with pressurized air from a syringe through a small bore "pilot tube". The air is injected into the proximal end of the pilot tube, which is a thin piece of tubing for its proximal half and a small diameter channel molded into the side wall of the tracheal tube for its distal half. The pilot tube terminates at its distal end within the inflatable cuff of the tracheal tube. The pilot tube has a one-way valve at its proximal end to maintain pressure within the system.
To prevent the leakage of air from between the inflated cuff and the tracheal wall during mechanical ventilation, the pressure in the cuff must be equal to or greater than the peak inspiratory pressure within the airway. Peak inspiratory pressures are only achieved for 10%-25% of the ventilatory cycle but may be as high as 50 mm of mercury. Since the pressure within the standard cuff is static, the cuff pressure must be maintained at this relatively high pressure (equal to or greater than peak airway pressure) throughout the ventilatory cycle, to prevent leaks during the highest pressure portion of the cycle.
The cuffs containing relatively high static pressures transmit that same pressure to the adjacent tissue of the wall of the trachea. As cuff pressure exceeds the capillary blood pressure of the tracheal tissues, normally 25 mm of mercury, tissue ischemia or inadequate blood flow occurs. Prolonged ischemia can cause varying degrees of injury ranging from mild erosion of the mucosa, to destruction of the tracheal cartilage rings, to segmental tracheomalacia with dilation of the trachea. Even more dramatic is full thickness erosion which may result in perforation of the innominate artery anteriorly or perforation of the esophagus posteriorly. Late complications of tracheal stenosis, from mild to incapacitating obstruction are noted in the majority of patients requiring long term ventilatory support. Again, the problem is not with the pressure itself but rather that the pressure is applied in a static form for a prolonged period of time. Short term occlusion of blood flow does not damage most tissues but prolonged occlusion causes tissue ischemia and cell death.
During surgery the problem of excessive cuff pressures is exacerbated because the anesthetic gas nitrous oxide diffuses through the cuff material and into the cuff. The volume of gas within the cuff can more than double during an operation because of nitrous oxide diffusion. Obviously nitrous oxide diffusion into the cuff increases the pressure within the cuff and therefore the pressure against the tissue of the tracheal wall is also increased. This slowly increasing cuff pressure may not be detected by the clinician.
To prevent tissue damage caused by prolonged pressure from the cuff, doctors and nurses may periodically deflate and re-inflate the cuff. However, this procedure is rarely done frequently enough or for a long enough period of time to allow adequate reperfusion of the tissue. Therefore, this procedure does not dependably prevent tracheal wall ischemia. It is also a nuisance for the clinicians.
A variety of pressure regulating devices have been developed to attach to the pilot tube to regulate cuff pressure. These include pop-off valves, chambers and balloons to visualize the amount of pressure and automatic inflator/deflators. None of these devices have reliably solved the cuff pressure problem.
The second practical problem with the present tracheal tube and inflatable cuff design is that the pilot tube assembly and valve requires many parts with many connections, resulting in many chances for failure, more assembly time and higher manufacturing costs. Because the pilot tube and cuff are a closed, relatively high pressure system that must maintain its integrity for hours-to-days, any leaks or manufacturing defects will result in cuff deflation and failure of the tube. The need for high levels of quality control during the manufacturing process of this life supporting device obviously adds to the cost of the device. In addition to occasional manufacturing defects, leaks frequently develop during prolonged use, necessitating tube replacement. Tube replacement in a critically ill patient can itself be a life threatening procedure.
These many problems can be eliminated or at least minimized by abandoning the use of the traditional statically inflatable cuff as the sealing means. Clearly, there is a need for an improved sealing means for use with tracheal and tracheostomy tubes.